GrowthThe growth rate of Arctica islandica is very slow and highly variable. Internal growth bands visible in cross-sections of the valves and hinge plates of Arctica islandica are deposited annually, possibly in response to the reproductive cycle of Arctica islandica (Thompson et al., 1980a). The presence of these annual growth rings on the shell of Arctica islandica allows individuals to be aged. The following ages have been recorded:

in New Jersey, (U.S.A) specimens as old as 53 years (13.6 cm) were recorded (Ropes & Murawski, 1983);

in Georges Bank, (U.S.A) specimens as old as 93 years (13 cm) were recorded (Ropes & Murawski, 1983);

whereas, a specimen 10.7 cm in length was estimated to be 221 years old in Massachusetts (U.S.A) (Ropes & Murawski, 1983).

It has been suggested that maximum growth occurs between 3 - 7 years of age and that growth slows down after 15 years (Thompson et al., 1980b). Individuals of similar size may vary greatly in age. For example, individuals ranging in age from about 50 - 179 years showed little discernible difference in mean length (Ropes & Murawski, 1983). Growth rates may be reduced at high densities and are also dependent on the temperature. Arctica islandica also exhibits geographical variation in growth rates. For example:

in Whitsand Bay, (UK) individuals grew on average 0 -1.5 mm/yr.;

whereas off the coast of Long Island, (U.S.A) individuals grew 0.56 mm/yr. in 1970 and 11.7 mm/yr. in 1980; and

specimens from New Jersey grew an average of 1 mm in 1.6 years (Kennish et al., 1994; cited in Cargnelli et al., 1999a).

Conditions required for fast growth, large size and longevity in this species are unknown (Ropes & Murawski,1983).

AbundanceIn the southeast North Sea, the average density of adults >10 mm in length was 7 specimens per 100 m². The highest abundances of spat and adults were recorded in the central regions of the North Sea with 21 individuals >10 mm per 100 m² (Witbaard & Bergman, 2003). The northern North Sea (Fladden Grounds) was dominated by juveniles with 28,600 individuals per 100 m² (Witbaard & Bergman, 2003). In Kiel Bay, Baltic Sea, recorded abundances were 50 /m² but, no size was recorded (Rees & Dare, 1993).

Biomass / ProductionIn major areas of the northwest Atlantic the standing stock was (expressed as wet meat weight) over 119,000 km², an average of 10 g/m², with a maximum (over 19,000 km²) of 16 g/m² (Murawski & Serchuk, 1989; cited in Rees & Dare, 1993). The total live shell weight /flesh weight ratio was not stated but was presumed to be 2:1 or 3:1). In Kiel Bay, the annual production was estimated to be 15 g ash free dry weight (AFDW) / m² and the biomass was estimated to be 44.4 g AFDW /m² (Rees & Dare, 1993).

RespirationArctica islandica like many other bivalves is able to respire aerobically and anaerobically. The change from aerobic to anaerobic respiration in bivalves usually occurs when oxygen levels in the water are reduced. However, Arctica islandica may experience periods of self induced anaerobiosis (Taylor, 1976).

Arctica islandica has only short siphons and siphonal contact with the water cannot be maintained when they burrow several centimetres beneath the sea bed. When this occurs they are able to respire anaerobically. No obvious rhythmic pattern has been detected but these periods of inactivity can last up to 10 days (Taylor, 1976). The heart rate of Arctica islandica after long periods of shell closure usually takes over 20 hours to return to normal (in Mytilus edulis this is accomplished in a few minutes). This behaviour in Arctica islandica is apparently self induced (Oeschger, 1990) since no stimulus that initiates either burrowing or a return to the surface has been identified. It was suggested that this behaviour is a likely response to hypoxic conditions however, previous studies showed that Arctica islandica was readily able to deal with hypoxic conditions. Other suggestions for this self induced burrowing behaviour included saving energy and a reduction in the risk of predation, as Arctica islandica would be less accessible to potential predators such as large starfish. However, more knowledge of their ecology is required (Taylor, 1976).

Arctica islandica is distributed within the North Sea north of 53° 30`N and along the southern and eastern borders of its range is limited to depths below 30 m (Witbaard & Bergman, 2003). Similarly, the optimal temperature for Arctica islandica was estimated to be 6-16°C, while the inshore limit of their distribution in the eastern USA was reported to be the 16°C bottom isotherm (Cargnelli et al., 1999a). Nicol (1951; cited in Holmes et al., 2003) stated that Arctica islandica occurs at depths ranging from 4-482 m, although it is commonly found between 10-280 m.

Sexual maturity Sexual maturity is reached in Arctica islandica at a later age than has been reported for other bivalves. The age at sexual maturity in Arctica islandica was reported to vary between 5 and 11 years and may be dependent upon growth rate and locality (Thorarinsdóttir, 1999) . The mean age of sexual maturity in Nova Scotia was estimated to be 13.1 years for males and 12.5 years for females (Rowell et al. 1990; cited in Cargnelli et al., 1999a). However, Thompson et al. (1980b) reported immature individuals ranging in age from 4 -14 yr., at shell lengths ranging from 2.4 - 4.7 cm. It was suggested that immature Arctica islandica physiologically mimic the reproductive cycle of adults without providing gametes but very little research has been done on annual cycles of reproduction in juveniles (Thompson et al., 1980b).

Gametogenic cycleThorarinsdóttir (2000) examined the gametogenic cycle of Arctica islandica from Iceland and suggests that there are five phases in Arctica islandica's gametogenic cycle (Thorarinsdóttir, 2000).

Early Active Phase. In females, oogonia appear and are embedded in the alveolar walls, often before the gonads are completely empty of ripe eggs. The shape of the oocytes may be square, triangular, cylindrical or hemispherical. In males, darkly stained spermatogonia are seen in the thickened alveolar walls but no spermatozoa are present.

Late Active Phase. In females, oocytes are larger and most are free from the membrane of the follicles. In males, secondary spermatocytes are numerous and the surrounding cytoplasm is irregular in shape and sperm appear in the lumina.

Ripe Phase. Follicles are extended and the gonads are full and ripe. In the female gonads the follicle contains almost exclusively free large ripe eggs. In males spermatozoa occupy the largest portion of the follicular spaces, while cells of early stages of spermatogenesis are few and confined to the area near the follicular walls. The sperm then form dense masses in the alveoli.

Partially Spent Phase. Gametes are discharged. In females, few large ripe oocytes are free in the lumina of some follicles but others are void of ripe oocytes. Oogonia and early oocytes may appear in the contracted and thickened walls. In males, substantially less spermatozoa are contained within the centre of the follicles. Most gonads are completely void of sperm.

Spent Phase. The follicles contain few undercharged eggs or sperm. Sometimes oocytes and primary spermatogonia begin to develop before the old ones are fully discharged (Thorarinsdóttir, 2000).

Investigations on Arctica islandica have shown that the timing and duration of events in the gametic cycle can be highly variable between years, which may reflect both environmental and endogenous factors (Thorarinsdóttir, 2000).

SpawningSpawning is protracted. Loosanoff (1953) reported spawning off Rhode Island between late June or early July when water temperatures reach 13.3 °C, although not all individuals reach ripeness at the same time. The majority of individuals completed spawning by early October. The optimum salinity range for the existence and reproduction of Arctica islandica is between 31.0 - 32.8 ppt (Loosanoff, 1953). However, the spawning period varies with location, for example another study reported spawning between May to November off Rhode Island, while spawning was reported to occur between September and November, and sometimes persisting into January off New Jersey, and between July to September off Nova Scotia (see Cargnelli et al., 1999a). Comparable data for northwest European populations is scarce. Attempts to ripen specimens of Arctica islandica out of season in the laboratory have had no or very limited success (Loosanoff, 1953; Landers, 1976). The average size of a fertilized egg is 75-95 µm (Loosanoff, 1953; Lutz et al., 1982).

Larval Settling TimeThe settlement of larvae may occur over several months and is believed to occur throughout the adult distribution ranges.

Recruitment The recruitment of this bivalve is considered as very sporadic (Thorarinsdóttir, 1999). Age frequency distributions from a population in Iceland showed recruitment increased at approximately 20 year intervals. A population of the Atlantic coast of the United States did not show any sign of recruitment over a 10 year period (Thorarinsdóttir, 1999).LongevityArctica islandica is exceptionally long lived. Growth is relatively fast during the juvenile stage and then slows down (Cargnelli et al., 1999a). Counts of internal growth lines suggested ages of 200 years for individuals living on the U.S. Atlantic coast (Ropes, 1985)